This invention relates to burners and heaters and stoves including such burners and, in particular, to multifuel burners, stoves and burners, such as multifuel stoves utilized for cooking during military field operations.
Earlier U.S. Pat. No. 6,450,801 assigned to Teleflex (Canada) Limited discloses a much improved liquid fuel stove apparatus compared with earlier equipment of this nature utilized for such purposes as cooking during military field operations. Such portable stoves are used in range ovens, steam tables, tray ration heaters, field sanitation equipment and stock pot heating racks. These appliances perform functions like roasting, grilling, broiling, frying, heating water and other liquids and baking. The stoves may operate under difficult field conditions and therefore must be reliable in operation and be capable of performing to a high level in order to meet the demands of the users.
The unit described above has a fuel delivery block which combines a series of different functions such as:    atomizing the fuel by mixing it with the compressed air;    opening and shutting off the fuel passages in accordance with operational requirements;    redirecting compressed air for atomizing the fuel or refueling;    accommodating the ignitor for starting the burning process;    supporting the flame sensor which maintains safe operations;            regulating the fuel flow from low to high, infinitely, relative to the knob position; and            maintaining a correct air/fuel ratio throughout the complete range of heat output for efficient and safe combustion.
In one embodiment, for example, the heater had a flow rate adjustable from 1.1 gallons per hour to 2.6 gallons per hour. For some applications, such as slow simmering, the minimum flow rate provided is too high.
In earlier multifuel burner units of the type described above, the fuel flow rate is controlled by a needle valve having an orifice and a cone-shaped needle on the end of a threaded stem. When the needle valve stem is turning on the thread, rotational motion transfers into linear displacement of the needle tip inside the orifice. It changes the opening area and, correspondingly, the fuel rate. The needle valve stem is attached to the knob by a collet which grasps the stem shank when the collet nut is tightened. Rotation of the knob is limited by two positive stops pressed into the body of the fuel delivery block and has an approximately 300° angle of rotation from high to low knob position. For example, when the knob is placed in the low position and the collet nut is released, it is possible to adjust the minimum fuel flow rate by turning the needle valve stem through the opening in the knob. Then tightening the knob, the fuel delivery block is adjusted to the required fuel flow setting (high flow, as well as the in between fuel range are defined by the profile of the needle). The knob also has two plastic tabs which are inserted into two slots of the inner ring of the potentiometer. The potentiometer is part of the power circuit of the compressor. When the knob turns, it also turns the inner ring of the potentiometer and changes its resistance. It activates the PWM (pulse width modulation) circuit and, as a result, compressor voltage changes. The stove electronic controller is programmed in such a way that, for each position of the potentiometer, it provides a certain voltage to the compressor.
In theory it would seem possible to reduce the output of the heater further utilizing the needle valve and the potentiometer. However in practice the PWM control circuit cannot be used to reduce the air flow rate further because the compressed air pulsates at low compressor speeds, causing emissions to go up considerably due to bad combustion. In the embodiment described above, for example, the low-end output of the compressor is 5 psi, while the high-end is 9 psi. Achieving a lower output would require a compressor output of approximately 2 psi. The earlier embodiment is not capable of such a low compressor output for the reasons discussed above.
Accordingly it is an object of the invention to provide an improved burner, heater and stove of the type described above but having improved simmering capabilities.
It is also an object of the invention to provide improved burners, heaters and stoves of the type described above, which are capable of operating at reduced fuel flow rates while maintaining air flow rates at a level for proper combustion.